Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Various methods may be used in implanted bionic devices to minimize power consumption. In some examples, some variety of time division multiplexing (i.e., time slotting) such as TDMA (time-division-multiple-access) scheme can be employed with pre-arranged synchronization between implants and non-implanted devices. In some cases, implants can be inductively powered by RF fields. In other cases, timeslots can be assigned to implants, and brief messages can be exchanged between the implants and the control units with some variety of communication that occurs at a regular cadence. The implants may be configured to rely on internal time constants such as resistor-capacitor based timing (i.e., RC timing), which may necessarily use very inefficient signaling means since time and frequency needs to be well known for most modern communication schemes.
In some cases a quartz crystal can be used as part of a time base circuit in an implant device. In such cases, the implant device's timing circuits may be implemented in a number of schemes such as a tuning fork or a shear mode resonator type of timing scheme. A typical tuning fork scheme using a quartz crystal may operate with a frequency of about 32,768 Hz. At these types of frequencies, a typical implant may use power on the order of nano-watts, but the implant may not provide an appropriate reference frequency for use with VHF/UHF transceivers, whose synthesizers typically may require more rapid updates.
The communication and time-keeping power requirements of tiny implanted devices may determine their useful battery life. For example, some of the power consumed in an implant device is determined by the transmit/receive power required for operation at the specified carrier frequencies, while additional power consumption requirements may be determined by the regularity or irregularity (i.e., intermittent periodicity) of communications.
Some implantable electronic medical devices may operate without the benefit of a quartz crystal. Unfortunately, since communications generally require both accurate scheduling and accurate frequency control, these types of implant devices may generally be unable to support efficient communications.
Some attempts have been made with inductively powered implant devices that use either the frequency of the inductive power or a multiple or integer fraction of the power frequency as a time base. Although these designs may be functional, such designs are typically very limited in capabilities, and cannot be practically used. For example, an inductively powered implant device may not be used at multiple locations on a patient, and may require wearing inductive power sources.
Some implantable devices may be configured to use shear mode quartz crystals with resonant frequencies in the MHZ range. In time base circuits (e.g., oscillators), these shear mode quartz crystal devices may require significant minimum operating currents to maintain oscillation (e.g., on the order of a few micro-amps). When such time base circuits are operated from low power, they may produce noisy clock signals that can adversely impact various circuits in the implant. In addition, the tiny sizes required for implantation may necessitate the use of tiny “strip” type resonator circuits such as AT cut quartz crystal strip resonators. Such small scale strip type resonators typically exhibit modest Q values compared to larger quartz resonators. The present disclosure appreciates that it may be difficult to achieve satisfactory time bases in implant devices that use miniature strip resonator topologies such as AT cut quartz crystal oscillators.
In some examples, a watch crystal (e.g., a quartz crystal) can be utilized in a tuning fork resonator type of time base circuit. One problem with the simple use of a watch crystal is that high resolution timing may not be supported by the use of such a time-base. If a tuning fork resonator is used as the frequency reference of a VHF or UHF radio transceiver, then the update rate will be slow and therefore the loop bandwidth will be small for any phase locked loop circuits that may be built using the tuning fork as a reference. In this case, phase noise performance at radio frequencies will be mediocre, and synthesizers start up and settling times will be long, of the order of milliseconds at least. This long settling time makes it difficult to operate the radio communications parts of the bionic implant efficiently in a short burst.
Micro-Electro-Mechanical System (i.e., MEMS) type resonators may be desirable in very small devices, since they are very small and may even be constructed as part of an integrated circuit that may be used for other purposes. The present disclosure appreciates that there are several difficulties associated with MEMS type of resonators.
In some examples a MEMS type of resonator may be built on silicon. One problem appreciated in the present disclosure is that the resonant frequency for a MEMS type of resonator built as parts of an integrated circuit tends to have a large temperature coefficient. The temperature coefficient is largely a consequence of the fact that the MEMS resonator will typically be built from silicon, where silicon has a large temperature coefficient of mechanical modulus. This temperature coefficient makes MEMS resonators difficult to use as accurate timing or frequency sources even given the narrow temperature range expected in a bionic implant.
In some additional examples, a MEMS resonator may be built on silicon without the use of a piezoelectric material. This type of MEMS resonator may need significant polarizing voltages to achieve reasonable electromechanical coupling coefficients.
In some further examples, a silicon based MEMS resonator such as silicon bulk acoustic resonators (i.e., SiBAR) uses the thickness mode, which may have desirably high Q values but also can result in the generation of low phase noise signals. An example SiBAR device is described by H. M Lavasani, A. K. Samarao, G. Casinovi and F. Ayazi in “A 145 MHz Low Phase-Noise Capacitive Silicon Micromechanical Oscillator”, IEEE International Electron Devices Meeting, pp. 675-678, December 2008; which is hereby incorporated by reference in its entirety.